Machining of various metals to form rough or finished end products has long been accomplished using any number of machining and/or cutting techniques including milling, turning, facing, boring, drilling, grinding, forming, shaping, planing, threading, grooving, etc. Each of these machining and/or cutting techniques involves significantly different processes, CNC machines and holders, cutting orientations and angles, entry angles, lead angles and cutting forces, cutting surfaces and inserts, pitches, and numerous other factors. In addition, each of these various machining and/or cutting techniques provides significantly different finished product in terms of use, shape, look, quality, finish, surface texture, surface roughness or smoothness, stress and cracking points, fatigue points, depth of cut, uniformity of cut, and numerous others features. For these reasons, tooling is generally technique specific.
Although milling and turning tools, and the cutting inserts used therewith, are sometimes similar in general appearance, the two processes are different in many ways above and beyond the obvious difference in that, in milling, the cutting insert generally moves and the workpiece is generally fixed while, in turning, the workpiece generally moves and the cutting insert is generally fixed. As is well known in the machining art, the art of milling versus turning involves significantly different factors, concerns, benefits and difficulties.
In milling applications, it is well known that cutting by each edge is intermittent as multiple inserts or edges are available on the mill and all sides of the mill are not in contact with the workpiece at all or substantially all times throughout the milling process. As a result, each insert edge is subject to periodic impacts during milling as it makes contact with the workpiece, is stressed and heated during cutting, and then rests and cools until the next impact. Thermal and mechanical fatigue result from this cycling impact and heat. These are a few of the difficulties encountered in milling that are not encountered in turning, or are encountered in such a lesser degree that these are not recognized as relevant. In contrast, turning typically involves a continuous or substantially continuous contact and heating thereby involving a completely different set of design and use factors and concerns.
For these and other reasons, the design of cutting inserts for milling is independent from the design of inserts and cutting edges for other machining such as turning, boring, slotting, etc. One of the current thrusts in milling technology today is a desire to perform as many milling steps with the same mill as possible, that is to be able to perform as many of the following as possible: plunging, ramping, circle interpolating, facing, and end milling with the same mill. Various mills on the market currently attempt to perform at least a few of these steps with an end result of a rough to semi-rough finish. An example of one of these mills is the RPF line of mills from Kennametal such as the one disclosed in U.S. Pat. No. 5,542,795 and those shown in various catalogs including Kennametal Milling Catalog No. 5040 on pages 4 and 182-189 (1995). These RPF mills function to ramp, plunge and/or face mill but typically only provide a rough or semi-finished surface.
One of the current concerns in milling is the stepped or wavy surface on the milled surface resulting from current technology, specifically in the areas of face and end milling. This stepped, uneven or otherwise irregular finish is typically a result of the type, position and location of the cutting inserts. This stepping is particularly relevant in attempts at all-in-one mills, that is mills capable of three dimensional milling, including plunging, ramping, circle interpolating, facing, and end milling with the same mill.
Cutting inserts that are not properly aligned along the flutes of the mill will provide such stepping. For example, on helical mills the inserts must be aligned to follow the curved contour of the helix and any misalignment results in stepping or other irregular finish such as a groove or lip where one insert is radially outward further than adjacent inserts. In addition, cutting inserts that have substantially rectangular configurations (that is, with long, flat cutting edges) will provide such stepping. This stepping is caused by the rectangular configuration of the insert removing excessive material from the workpiece at or approaching the corners of the inserts thereby causing stepping. In contrast, cutting inserts that have an arc-like periphery along its entire cutting surface (radiused corners that extend acrcss the whole cutting edge) remove excess material from the workpiece along the mid-section of the cutting insert, thereby causing valleys.
According to the milling users, this stepped surface is often unacceptable, particularly when a finished, not rough, surface is desired. This stepping is unacceptable because stress points form at each or some of the inner and outer edges of the steps as these edges are often well defined. This forces the milling user to thereafter finish mill the surface which is an additional time consuming and expense-adding step.
Various improvements have been invented in certain specific types of milling, such as finish milling, to diminish or attempt to overcome this stepping, although it is believed that none of these improvements have been provided on so called all-in-one mills. These improvements include rounding the four corners of rectangular cutting inserts as is shown in the above referenced U.S. Pat. No. 5,542,795. Specifically, the rounding of the corners as achieved in the prior art results in a single radii defining the transitions between sides of the insert. This single radii rounding reduces or eliminates the stepped surface with well defined edges, grooves, ridges, etc.
However, these single radii rounded corners and/or the inability to properly position and align all of the inserts along the flute or other surface on the mill still cause the insert to bite excessive material from the workpiece during milling thereby creating a wavy surface having smooth peaks and valleys. The wavy surface is an improvement over the undesirable stepped surface but still presents problems in many applications. Thus these improvements have either reduced the significance of this undesirable stepping, or provide a less significant wavy finish that includes some peaks and valleys but no or minimal sharply defined steps. Milling users have indicated that in many milling applications this rough or semi-finished surface is acceptable with its wavy surface so long as the unacceptable stepping is eliminated since it is the sharp edges of the step that are the significant stress points. For this reason, RPF type cutters have become very popular.
Other cutters have been developed to remove this waviness but such cutters are unique in application. These specialty cutters are for surface finishing only. That is, these cutters are not capable of ramping, plunging, facing and end milling. An example of such a finishing cutter tool is found on Pages 178-179 in Kennametal's Milling Catalog (No. 5040 from 1995). In application, often a drill or other machining device is needed to initially drill a hole, followed by a ramping, slotting or other cutting device for milling out the desired area from the hole, before such a finishing cutter is used. This adds significant time and cost to the operation as multiple milling cutters are needed.
In many applications a rough or semi-finished surface is not sufficient, but the alternative of using multiple milling cutters is undesirable for time and cost reasons. Therefore, it is desirable to improve on the removal of all stepping and as much waviness from the finished milled surface while still providing a milling cutter capable of performing three dimensional milling, that is as many of the needed steps as possible including ramping, plunging, circle or helical interpolating, facing and end milling.
Such improvement continues to be sought by mill users in various industries since such reduction or complete elimination of all stepping and waviness will increase the overall life of the metal end product by reducing and/or eliminating stress, fatigue and other undesirable forces working against the metal end product. In conjunction with this, if such milling cutter could perform all or substantially all of the typical milling steps including ramping, plunging, circle interpolation, facing and end milling, then significant cost and time reduction would also be achieved. Finally, if all such milling steps could be achieved by one milling cutter, the cutter must have a reasonable life as these various steps involve different concerns and factors including the subjecting of the cutter to large axial, radial, and tangential forces, possibly simultaneously.